The word “variator” is used herein to refer to a device which transmits rotary drive at a continuously variable ratio. Variators are particularly, but by no means exclusively, applicable in motor vehicle transmissions.
The best known form of rolling-traction type variator uses at least two co-axially mounted races having opposed faces which are shaped so that the races together define an approximately toroidal space. At least one roller is positioned in the space between the races and runs upon their shaped faces to transmit drive from one to the other. Changes in the inclination of the roller are associated with changes in the relative speeds of the races, and hence in the drive ratio provided by the variator.
The changes in roller inclination associated with changes in drive ratio will be referred to herein as “precession” to distinguish from other rotary motions of the roller, such as its rotation about its own axis.
Some mechanism is needed to control roller inclination, and the prior art contains numerous examples. Typically such mechanisms do not act by directly applying a torque to the roller's mountings. Instead, the roller is mounted in such a manner that displacing it causes it to steer itself; due to the forces exerted on it by the races, to a new inclination. The steering effect arises because the roller seeks a position in which its own axis coincides with the common axis of the variator races, since in any other condition the motion of the roller is non-parallel to that of the races in the area where they engage with each other. The control mechanism serves to regulate the roller's displacement.
Examples of such mechanisms are found in many of the applicant's prior published patent cases including PCT/GB03/00259 (WO 03/062670). In most, the displacement needed to cause the roller to steer itself is along the circumferential direction (about the common axis of the variator races) and, by allowing the rollers to process about an axis which is inclined to the radial plane, a relationship is established between roller displacement and roller inclination. An actuator is provided for urging the roller along the circumferential direction and so influencing (1) its displacement, and (2) the variator ratio.
Such mechanisms lend themselves to “torque control” of the variator. The concept is known in the art, but will be briefly explained. More conventional “ratio controlled” transmissions are constructed such that they receive some form of input indicating a required drive ratio, and then adjust themselves to provide it. That is, the drive ratio is directly set. By contrast, in a torque controlled transmission it is torque which is directly set. Changes in ratio result from application of the torque to inertias at the input and output, and the variator automatically accommodates such changes. The sum of the torques acting on the variator races will be referred to herein as the reaction torque, since it is the torque which must be reacted to the variator's mountings. The reaction torque is referred to the rollers, and so through their associated actuator(s) to the variator casing. Hence by setting the actuator force, the reaction torque itself is directly set, since (neglecting roller acceleration) the forces exerted on each roller by the actuator and the races must be equal and opposite. Control over the transmission is exercised by controlling actuator force—and hence reaction torque—not variator ratio.
The most widely adopted control mechanism uses a respective hydraulic piston/cylinder type actuator for each roller, the piston being coupled through a piston rod to a carriage carrying the roller. However, a quite different type of mechanism is described herein, in which the variator has a sun and a ring and the roller carrier engages with both. Relative rotation of the sun and ring causes a tilting motion of the carrier, and hence causes the roller to steer itself to a new orientation. In this type of arrangement, driving the sun is problematic. Some coupling needs to be made to the sun for this purpose and in principle this could be made along an axial direction—e.g. through some sleeve extending along the variator's shaft—or along a radial direction—e.g. through an arm extending through the toroidal cavity. The former option creates design difficulties. The latter is problematic because the arm would foul the rollers and/or their carriers as they move back and forth.